This invention relates to a strip line cable comprising a strip or belt-like dielectric material and elongate and narrow electrical conductors which are arranged in parallel to one another in sets of pairs facing each other on both sides of the dielectric material. The strip line cable of this device has improved electric signal transmission characteristics.
Transmission lines for microwave and millimeter wave transmission include waveguides, coaxial lines, and strip lines. Being made up of two conductors, the coaxial lines and strip lines are capable of transmitting direct current as well as alternate current. Because of this capability, they are in general use for electric signal transmission. Particularly, the strip line is attracting attention for use as a signal transmission line for computers and other information processing machines because it holds many conductors in a compact size.
It is generally known that the strip line has improved electric signal transmission characteristics when the dielectric material used is one which has a low dielectric constant and dielectric loss, together with a minimum of frequency dependence. In other words, if the dielectric material has a low dielectric constant, it is possible to reduce the size of the strip line, although its characteristic impedance can remain the same, and the rate of signal transmission increases. If the dielectric material has a low dielectric loss, it is possible to reduce the attenuation of signals. Also, if the dielectric material has a low dielectric constant and dielectric loss with a minimum of frequency dependence, the strip line is capable of transmitting pulse signals with a minimum of distortion.
Thus, the dielectric material used for strip lines is required to have a low dielectric constant and dielectric loss with a minimum of frequency dependence. A known example of such a material is a microcellular material of open-cell structure made of a crystalline organic polymer. This material has such an internal structure that a large number of nodes are connected by minute fibrils and a large number of voids are formed among the nodes and minute fibrils. An example is expanded porous PTFE produced according to the method disclosed in U.S. Pat. No. 3,953,566. Other materials used include polyolefins (polyethylene, polypropylene, polystyrene, etc.), polyamides, polyesters, and fluoroplastics (polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexapropylene copolymer resin (PFA), and tetrafluoroethylene-perfluoroalkylvinyl-ether copolymer resin (PFA), and tetrafluoroethylene-ethylene copolymer (ETFE)) which are made porous by the stretching process, salt leaching process, or solvent evaporation process.
The present inventors noticed that a porous crystalline polymeric material as mentioned above has a low dielectric constant and dielectric loss with a minimum of frequency dependence, which is required for the dielectric material of strip lines. (See Japanese Patent Laid-Open No. 158502/1981 and Japanese Utility Model Laid-Open No. 5820/1984 issued to the present inventors.) When the dielectric material interposed between the pairs of conductors facing each other was made of a porous plastic material having a low dielectric constant, it was possible to improve the performance of the transmission line, particularly to increase the rate of signal transmission. However, it was found that there was a limit in such improvement. In order to further increase the rate of signal transmission, the present inventors carried out extensive studies and conceived the improved structure of strip line according to this invention.